1. Field of the Invention
The present invention relates to an image processor, a semiconductor integrated circuit, and a method for processing an image, capable of executing black stretching by a digital system.
2. Description of the Related Art
A video signal for displaying an image on a display includes a luminance signal, a color signal, and a synchronizing signal. An image processor such as a television set has a black stretching circuit for improving the contrast of the image and for emphasizing black color. The “black stretching” refers to an operation of amplifying the level of the luminance signal equal to or less than a fixed luminance level (start point) toward the black side of the luminance level. The black stretching circuit is arranged as a feed back circuit, and includes a black stretching amplifier for generating a black stretched signal from the luminance signal. The black stretched signal is compare with a reference detection level. A signal component in the black stretched signal equal to or less than the reference detection level is detected as a black level. The gain of the black stretching amplifier is controlled so that a ratio of a black level component to an image in a fixed period corresponds to a setting ratio.
With the progress of digital technology, the black stretching circuit has been digitized. Since the analog circuit can control the gain continuously, a black stretching amplifier arranged as the analog circuit can achieve fine characteristic by use of feed back technology. However, the gain of a black stretching amplifier arranged as a digital circuit is discrete. Therefore, the gain of the black stretching amplifier is controlled by use of a gain step-size. The “gain step-size” refers to a value obtained by dividing the maximum gain of the black stretching amplifier by a fixed value. When the black stretching is executed by a digital system, a ratio of the black level to an image in a display (hereinafter referred to as “black color ratio”) is detected, and the gain of the black stretching amplifier is controlled based on the detected black color ratio.
With respect to the black stretching amplifier arranged as a digital circuit, the gain is controlled so that the black color ratio can reach to the setting ratio. When the gain is suddenly changed into the fixed gain, the change become clear to the human eye. Then, the step-size of the gain change is so selected such that the change of the image does not become noticeable. When the step-size of the gain change is too small, the time for increasing the black color ratio up to the setting ratio increases.
When an image level in a display is uniform and little higher than the reference detection level, i.e., the entire display is a black color near the reference detection level, the black color ratio goes to 0%. When the setting ratio is 3%, the black stretching amplifier amplifies the luminance signal toward a black level, and increases the black color ratio. However, since the image level in the display is uniform, the black color ratio after the amplification goes to 100%. When the black color ratio goes to 100%, the black stretching amplifier suppresses the amplification, and decreases the black color ratio. As a result, the black color ratio goes to 0% again. As described above, image flicker occurs in the display because the black stretching amplifier repeatedly increases and decreases the gain. Therefore, the characteristics of the black stretching circuit of a digital system is worse than a black stretching circuit of an analog system because the black stretching circuit of the digital system is not able to control the gain continuously.